Potential Sulfate Reduction Rates within the Interstitial Material of Massive Sulfides of Middle Valley

Microbially-mediated sulfate reduction (SR) is –in general- the coupling of a biotic (e.g. acetate) or abiotic (e.g. hydrogen) reductant to sulfate as an oxidant for energy generation to support growth and reproduction.  At hydrothermal vents, numerous thermophiles (thrive at 45-80°C) and hyperthermophiles (thrive above 80°C) gain metabolic energy by reducing sulfur compounds.  In marine systems, sulfate is highly abundant and, as such, the reduction of sulfate is calculated to be among the most favored anaerobic metabolic pathway in hydrothermal conditions.   Despite the hypothesized ubiquity of SR at vents, few studies have directly measured SR within vents and relatively little is known about how environmental or ecological factors influence rates of SR.  Here, we measure microbial sulfate reduction rates by ³⁵S tracer techniques in crushed chimney material from three chimneys of Middle Valley on the Juan de Fuca Ridge and evaluate these rates in the context of the in situ geochemistry and biological makeup of each vent-hosted microbial community.  The rates of SR at middle valley were significantly different among chimneys and comparable to cited measurements of SR rates in hydrothermally influenced sediment.  SR rates ranged from 15.7 nmol g^-1 day^-1 (at Dead Dog at 60°C) to 2674 nmol g^-1 day^-1 (at Needles at 90°C). In all three chimneys the maximum rates of sulfate reduction at occurred at 90°C, suggesting that microbial communities are primed for hyperthermophilic SR.  The microbial biomass and community structure unique to each chimney seems to directly influence the magnitude and temperature dependence of sulfate reduction.